Refractive composition and method for producing the same



Patented Aug. 3, 1954 UITED STATES ATENT OFFICE REFRACTIVE COMPOSITION AND METHOD FOR PRODUCING THE SAME N Drawing. Application February 2, 1953, Serial No. 334,705

Claims. 1

This invention relates to monocrystalline strontium titanate and to methods of making and treating monocrystalline strontium titanate.

Monocrysta-lline strontium titanate is characterized by a high index of refraction (about 2.4) and a low reciprocal relative dispersion or 1 value (about 13). Large single crystals of strontium titanate may be made which are clear, transparent and glass-like and such crystals have important optical applications because the index is substantially higher and the reciprocal relative dispersion substantially lower than those of the densest flint glasses. Whether clear or colored, the crystals have important gem stone and other applications because of their high refraction and dispersion.

One broad use of optical materials having high refractive indices and high dispersion is in the manufacture of lenses and prisms. For example, materials having a high index of refraction are utilized in the construction of optical systems such as telescopic or high magnification microscopic objectives. Furthermore combinations of optical materials which individually possess widely variant optical properties are used to prepare achromatic lenses. High dispersion qualities are also useful for making prisms and the like. Highly refractive single crystal material is also useful for the preparation of ornamental objects such as gem stones. Synthetic gems of various types and colors are in great demand for both personal adornment and industrial uses.

An object of this invention therefore is to provide a monocrystalline glass-like composition which possesses exceedingly high index of refraction with a high dispersion or relatively low 1/ value. A further object is to provide a method for producing a monocrystalline composition suitable for manufacture of lenses, prisms, and partim ularly gem stones. Further to'provide for a monocrystalline mass having a color ranging from substantially water-white through blue to blueblack and to provide a method for making the same. These and other objects will become apparent from the following more complete description of the instant invention.

Broadly this invention contemplates a monocrystalline mass of strontium titanate having a color ranging from substantially water-white through blue to blue-black and a method for producing the same which comprises periodically introducing into an oxygen-hydrogen flame a powdered composition consisting essentially of strontium titanate and an efiective amountup to about 3.0% of a compound selected from the group consisting of oxidic compound of columbium and oxidic compound of tantalum and crystallizing the melted material as a monocrystalline mass in the form of a boule and subsequently subjecting the boule to an oxidizing atmosphere at temperature from about 650 C. to about 1700 C. to produce the desired color. It has been found that amounts of addition agents as low as about 0.005% are effective in changing the color of the strontium titanate crystal produced.

This application is a continuation-in-part of co-pending application Serial No. 252,906 filed October 24, 1951, now U. S. Patent No. 2,628,156, issued February 10, 1953, which describes and claims monocrystalline strontium titanate prepared without the addition of modifying or coloring agents. The process for preparation of the single crystals of strontium titanate is more fully described in co-pending application Serial No. 334,704 filed February 2, 1953, entitled: Refractive Material.

The term strontium titanate is intended to embrace both pure and substantially pure SITlOs,

the latter which contains impurities or added coloring or modifying agents either present or added which are of a nature and in amount so as to not aiiect the monocrystalline structure nor alter the desired color of the strontium titanate material produced. In most cases the impurities are held to a minimum and ordinarily will not exceed a few tenths of a per cent, and modifying,

or coloring agents are added in the amount necessary to produce the desired efiect.

In order to prepare a monocrystalline mass by co-fusing particles of strontium titanate, it is necessary to employ high temperatures. Strontium titanate melts in the neighborhood of about 2050 C. and it is therefore preferred to use an oxygen-hydrogen flame in order to obtain the necessary temperatures without the possible introduction of impurities.

It is convenient to drop the finely divided particles of the strontium titana-te composition into the area of intense heat formed by the oxygenhydrogen fiame, i. e. in or near the oxygen cone. Within this area the strontium titanate composi tion melts and as it cools below the melting point crystallizes as a single crystal on a base which holds the molten strontium titanate material. In order to start such a single crystal to form it is desirable to first form a seed and then gradually build up the amount of molten material on the seed to form the crystal. Such a procedure allows the crystal to build up upon itself gradually increasing in diameter until a boule or carrot-shaped single crystal of strontium titanate is formed. The size of the orifices of the oxygenhydrcgen torch determines the size of the intensely heated zone which in turn, determines the diameter of the carrot-shaped crystal produced.

It is preferred to employ a strontium titanate powdered material which is substantially free from objectionable or incompatible impurities which detrimentally would afiect the crystal structure. Starting material should be finely divided and fairly uniform in size. Such a material may be conveniently prepared by first reacting, titanium oxalate with strontium chloride to precipitate strontium titanium oxalate. After thoroughly washing the strontium titanium oxalate, it is heated at a temperature of at least 500 C. to remove the oxalate portion thus forming strontium titanate. Preferably this strontium titanate material is then crushed and ground to obtain finely divided material from which the strontium titanate single crystal is subsequently formed- Substantially any oxidic compound of columbium and tantalum can be added to the feed material, for example, the oxide or any oxidic com-' pounds which form oxides at the temperatures employed in the preparation of the strontium titanate boule. According to the present invention a small amount of these addition agents, for example, the oxide may be mixed with the strontium titanate starting material in any convenient manner for instance, by dry mixing. A more uni.- form mixture and therefore a preferred mixture may be prepared by adding compounds of the addition agent as a powder, slurry, or solution, to the strontium titanium oxalate prior to the heat treatment to remove the oxalate portion. The amount of addition agent to be added to the strontium titanate starting material is very small and should not exceed about 3.0% calculated as metal. oxide based on the weight of the strontium titanate.

This invention covers adding small amounts, i. e. effective amounts up to about 3.0% of oxidic compounds of either columbium or tantalum. The amounts of addition agents to be employed may be varied depending upon the desired final color. When employing strontium titanate containing various amounts of addition agents in the. feed material according to the instant invention, the boule as produced possesses a black color. After it is subjected to an oxidizing treatment, however, the boule changes in color to producev the desired color. The depth of color is dependent upon the amount of addition agents employed. It has been found that when more than about 0.05% of the addition agent is added to the strontium titanate feed material the crystal after the oxidation treatment remains substantially black in color.

When smaller amounts of the addition agent are employed, blue to light blue to substantially water-white colors are obtained after the subsequent oxidizing treatment. In order to show the effects of the various amounts of addition agents upon the color of the boule produced, the following table is presented showing the various percentage ranges which produce the various colors after the oxidation treatment.

The boule described in the abov table were prepared in the following manner. Feed materials consisted. of a mixture of strontium titanate and various quantities of either columbium or tantalum oxide. These feed materials were progressively fused by passing the powdered material through the flame. The fusions were carried out at temperatures between 2080 C. and 2150" C. The boule was grown on a pedestal as the fused material was progressively crystallized. As produced, the boules were substantially black in color. They were substantially carrot-shaped and had a more or less frosted outer surface. When split, the interior surface of the pieces is vitreous and shiny. These boules were subsequently subjected to an oxid zing treatment, in either whole or split form, at temperatures from 650 C. to 1700 C. for 12 hours to 180 hours. These oxidized crystals were then cut and polished to form beautiful blue to white gems and the lighter colors were used to prepare lenses, prisms, and the like which possessed high indices of refraction with high dispersion.

From the above description a monocrystalline glass-like composition has been prepared which possesses a high index refraction with a high dispersion. Such composition is suitable for the manufacture of lenses, prisms, and particularly gem stones. By the process of the instant invention glass-like compositions ranging in color from substantially water-white through blue to blue black are obtained.

While this invention has been described and illustrated by the examples shown, it is not in tended to be strictly limited thereto and other modifications and variations may be employed within the scope of the following claims.

I claim:

1. Highly refractive glass-like monocrystalline composition consisting essentially of strontium titanate and amount from about 0.035% to about 311% by weight of compound selected from the group consisting of oxidic compound of columbium oxidic compound of tantalum.

2. Highly refractive colored glass-like monocrystalline composition consisting essentially of strontium titanate and amount from about 0.005% to about 3.0% by weight of a compound selected from the group consisting of oxidic compound of columbium and oxidic compound of tantalum and characterized by a color ranging from substantially water-white through blue to blue-black.

3. Method for the production of a colored monocrystalline mass of strontium titanate which comprises periodically introducing into an oxygen-hydrogen flame a powdered composition consisting of strontium titanate and an amount from about 0.005% to about 3.0% by weight of a compound selected from the group consisting of oxidic compound of columbium and oxidic compound of tantalum crystallizing the melted material as a monocrystalline mass in the form of a boule and subsequently subjecting the boule to an oxidizing atmosphere at temperature from about 650 C. to about 1700 C. to produce the desired color ranging. from substantially water-white through blue to blue-black.

4. Composition according to claim 1 in which the oxidic compound is columbium. oxide.

5. Composition according to claim 1 in which the oxidic compound is tantalum oxide.

References Cited in the file of this patent UNITED STATES PATENTS Number 

1. HIGHLY REFRACTIVE GLASS-LIKE MONOCRYSTALLINE COMPOSITION CONSISTION ESSENTIALLY OF STRONTIUM TITANATE AND AMOUNT FROM ABOUT 0.005% TO ABOUT 3.0% BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF OXIDIC COMPOUND OF COLUMBIUM AND OXIDIC COMPOUND OF TANTALUM. 